Electroacoustic transducer



Feb. 25, 1947; B. M. HARRISON ELECTROACOUSTIC TRANSDUCER Filed Dec. 19,1939 2 Sheets-Sheet 1 INVENTOR.

Bef-fmm M. Harrison Feb. 25, 1947. B. M. HARRISON ELECTROACOUSTICTRANSDUCER Filed Dec. 19, 1939 2 Sheets-Sheet 2 Patented Feb. 25, 19472,416,314 ELECTROACOUSTIC 'raANsoUcna Bertram M. Harrison, NewtonHighlands, Mass., assignor, by mesne assignments, to Submarine SignalCompany, Boston, Mass., a corporation of Delaware Application December19, 1939, Serial No. 309,992

12 Claims. 1

The present invention relates to a submarine electroacoustic transducer.More particularly the present invention relates to a device forsignaling under water with compressional waves and especially fortelephone communication.

The features and objects of the invention as well as its constructionand manner of operation will best be understood from the followingdescription taken in connection with the accompanying drawings in whichFig. 1 represents a plan view of the device with the diaphragm removed;Fig. 2 is another view, the right half being a vertical mid-sectionalong the line 11-11 in Fig. 1 and the left half being an elevation witha portion of the diaphragm cut away; Fig. 3 shows a schematic diagram ofthe circuit arrangement of the structure as set forth in Fig. 1; Fig. 4shows a fragmentary view in perspective illustrating the mounting of theelectrodes and the electrical connector to the crystals; and Fig. 5shows the mounting of the apparatus of Fig. 1 on the deck of asubmarine. v

Upon the base member i which forms one end of the casing of the unitthere is mounted-a plate 2 spaced from the plate i by spacers 3. Uponthe plate 2 is mounted a hollow polygonal prism 4, here illustrated asa-hexagonal prism. The prism plate 2 and spacers 3 are held to the plateI by means of the screws 5. The side walls of the prism 4 are given athickness of one-eighth of the wave length of compressional waves in thematerial at the signaling frequency. The bottom of the prism is closedby the plate 2 while the top of the prism is closed by a memberpreferably made integral with the side walls of the prism.

' This has a thin edge portion 6 and a thicker central portion I whichalso has a thickness of one-eighth of the wave length of compressionalwaves in the material. The interior of the prism is left filled withair, or any other gas may be substituted or it may be evacuated, thejoint with the plate 2 being tightly sealed against the entrance of anyliquid.

On each side face of the prism there are mounted a plurality ofelectroacoustic elements such as the piezoelectric crystals 8,preferably Rochelle salt crystals. Each of these crystals has a lengthprojecting at right angles from the face of the prism equal toone-quarter of the wave length of compressional waves in the crystals atthe signaling frequency. In the construction illustrated the crystalsare arranged on each side face of the prism in a double row 8 and 8" asevident from Fig. 2. As many rows may be used as desired to give therequired energy output consistent with the desired beam pattern in theplane perpendicular with the direction of the rows. The crystals haveelectrodes secured to their wide faces. The crystals in each row arearranged in pairs, the pairs being separated from each other byinsulating members 9 having a. thickness small compared to the wavelength in water. In this manner the crystals of each pair are connectedtogether in series and I prefer to connect the several pairs together inparallel. This arrangement of connections may, however, be varied in anydesired manner as will be understood by those skilled in the art. Thecrystals and their separators are fastened to the hexagonal mass memberin any suitable manner as by means of cement. By extending theseparators part way along the sides of the crystals the separators helpto support the crystals and to make the structure sufliciently ruggedfor use under water.

The-crystals mounted on the several faces of the prism make it possibleto radiate and receive compressional wave energy in all directions,particularly in the plane parallel to the ends of the prism whichusually will be the horizontal plane. By using a hexagonal prism, thewidth of whose side faces is approximately equal to 0.89 times the wavelength in the water, the beam patterns of the radiation from the severalfaces will overlap somewhat to assure full coverage in all directions.

In order to be able also to transmit and receive in a vertical directionone or more additional groups of crystals Ill are mounted on the top ofthe prism on the member Only one row of these is shown but at least tworows will usually be necessary for uniform hemispherical radiation. Ason the sides, the crystals are arranged in pairs with supporting andseparating members ll between them.

In front of each group of crystals and spaced as close to the crystalsas possible is a diaphragm member made of a material having a specificacoustic impedance which is substantially the geometric mean between theimpedances of the crystal and of the outer water. The diaphragm memberhas a thickness of one-quarter of the wave length of compressional wavesof the signaling frequency in the material of which it is made. Thediaphragm member is preferably made of one piece as, for example, ofmolded macerated Bakelite and shaped to cover the entire crystal unitand form with the plate I a watertight housing for the same. Asillustrated, the diaphragm member I2 is in the form of a 3 1 hollowcylinder closed at one end and having its walls and end one-quarter wavelength in thickness. The Bakelite diaphragm is fitted into a groove l3in the plate I to which it is secured by the screws M, a soft rubber orother watertight gasket l5 being interposed. The interior of the memberI2 is dimensioned so that when in place on the member I the interiorwalls and end will be as close to the crystals as possible, but not incontact with them. In order to transmit com pressional wave energy toand from the crystals; the interior of the unit is filledwith a suitablecompressional wave conducting liquid such as oil, which may beintroduced through the aperture occupied by the plug l6. Leads forconnecting the crystals to a source of electric energy or to amplifyingand indicating apparatus may be carried out from the interior of theunit through a suitable watertight stuffing tube indicated at II.

It will be observed that each crystal, forming a one-quarter wave lengthvibrator, is mounted by one end upon a one-eighth wave length massmember which has the effect of a substantially infinite mass which willreflect back into the crystals substantially all 'the vibratory energyreceived from it. Furthermore, the free end of each crystal is matchedto the outer water by the interposition of the one-quarter wave lengthdiaphragm member l2 to which it is acoustically connected by the liquidwithin the unit.

As above stated, the device as a whole provides a plurality of planeradiating surfaces so dimensioned and positioned that the severalsurfaces formed by the groups of crystals produce beam patterns in themedium which overlap in such a manner that substantially uniformlyintense radiation and substantially uniform reception response isobtained in all directions in a plane normal to the radiating surfacesand fairly uniform in planes perpendicular to said normal plane. I havefound that this result can be obtained by making the effective width ofthe radiating surfaces approximately equal to but not substantiallygreater than 0.89 times the wave length of compressional waves'in thesignaling medium at the signaling frequency.

I with the necessary electrode thickness may oo- For example, for adevice adapted for operation at 24 kilocycles the total width acrosseach group of crystals including those on top of the device should beabout 2.13 inches. This, of

..course, holds only for the hexagonal arrangement herein shown. If morepower output is desired it will be necessary to make the crystal groupslarger and hence each group will produce a sharper beam. The hexagonalarrangement then becomes insuflicient for uniform cylindrical radiationand it will be necessary to increase the number of faces of thepolygonal figure. If uniform hemispherical radiation is required, it mayfurther be necessary to break up the top surface into a number ofangularly disposed surfaces. The rule to be observed is that theintensity of wave pressure produced by each of two adjacent effectiveradiating surfaces in directions at which the two intensities are equalshall be not substantially more than 3 decibels less than the maximumintensity produced by each surface, for then the resultant intensitywill be substantially the same in all directions.

To complete the example of a suitable arrangement as herein describedfor 24 kilocycle operation, the hexagonal mass element 4 may be made ofsteel 1.0 inch thick. The Rochelle salt crystals 8 may be 1.5 inches inlength and 0.25 inch thick by 1.0 inch in width; the separators 9together cupy spaces 0130.043 inch each. so that the total width acrossthe faces of the crystals in each row is 2.13 inches. The separation ofthe rows is 0.13 inch so that the span across the two rows is also 2.13inches. Finally, the diaphragm element l2 may be made-of moldedmacerated Bakelite 0.75 inch thick and the interior of the unit may befilled with castor oil. This unit will absorb approximately 50 watts ofinput power.

This device is particularly adapted to telephonic communication betweena disabled submarine and another submarine or a surface vessel. For thispurpose the device is preferably mounted on the outside of thesubmarine, for example, on its upper deck in a position where soundwaves will be free to travel to and from it in all directions when thesubmarine is submerged.

It will be understood by those skilled in the art that it is notessential to use piezoelectric crystals for the operating elements butthat onequarter wave length magnetostrlctive members may be used insteadif desired.

A futher feature of my device which should be noted is that each groupof crystals is completely shielded from waves approaching fromdirections to the rear of that group by virtue of the hollow massmember. Thus, if desired, by separately bringing out leads from eachgroup of crystals and providing suitable switching means, any one groupof crystals can separately be listened to while receiving. If the groupmost nearly facing the source of waves be selected, disturbing noisesarriving from opposite directions will be eliminated automatically. Alsoby virtue of this shielding effect it is possible to use the device fordetermining the general direction of a wave source by successivelylistening to the several receiver r p Fig. 3 illustrates schematicallyan arrangement whereby the operations described above may be carriedout. As illustrated in this figure the crystals 8 and 8' which arearranged on the prismatic faces are connected in groups 20, 2|, 22, 23,24 and 25. Each group may be connected, as indicated, in a seriesparallel connection by connecting the crystals 8 in series and thecrystals 8' in series and connecting the two groups in parallel. Thecable 26, as indicated in Fig. 3, may have connected across it both thesender 21 and the receiver 28 and each listening group may be connectedacross the circuit by means of the switches 29, 30, 3|, 32, 33 and 34.The top listening group of crystals l0 may also be connected across theline by means of. the switch 35 con.- nected across the cables 26. Itwill be seen from the above arrangement that any of the groups can beused singly or together with any other group and that if generallistening is desired or general transmission, all of the units can beconnected across the line.

In Fig. 4 which shows a perspective arrangement of a single group ofcrystals l0, 8 or 8' it will be seen that each crystal is provided withan electrode 36 on each side of the crystal, the center electrode 36serving for the adjacent faces on two crystals. An electrode connection31 is mounted on the plate electrode 36 and serves as a terminalconnection for the electrical circuit. The blocks 9 are mounted on thesides of each pair of crystals and serve as mounting and spacingelements between the crystals.

In Fig. 5 there is shown a method of mounting the unit as a whole on thedeck of a submarine. Here the unit 38 may be mounted in a fixed p0-sition on a projecting support 39 from the deck Having now described myinvention, I claim:

1. An electroacoustic submarine transceiver comprising a plate, a hollowpolygonal prism mounted on said plate, the walls of said prism beingone-eighth of a wave length in thickness at the signaling frequency,means closing the lower end of said prism, means supporting said prism:to said casing at said lower end, means closing the upper end of saidprism and having a thin edge portion and a central portion having athickness of one-eighth of a wave length, a plurality of one-quarterwave length Rochelle salt crystals having electrodes with electricalconnections attached thereto, the crystals being mounted by their endsupona plurality of thefaces and upon the thick portion of the meansclosing the upper end of said prism, a cup-shaped diaphragm of moldedmacerated Bakelite a quarter wave length in thickness secured by it edgeto said plate and positioned to enclose with said plate all of saidcrystals and having its interior surfaces which are opposite the freeends of said crystals only slightly spaced from them and a compressionalwave conducting liquid filling the enclosed space.

2. An electroacoustic submarine transceiver comprising a plate, a hollowhexagonal prism mounted on said plate, the, faces of said prism having awidth approximately equal to 0.89 times the wave length of compressionalwaves in the signaling medium at the signaling frequency and the wallsof said prism being one-eighth of a wave length in the material inthickness at the signaling frequency, means closing the lower end ofsaid prism, means supporting said prism to said casing at said lowerend, means closing the upper end of said prism and having a thin edgeportion and a central portion having a thickness of one-eighth of a wavelength in the material, a plurality of one-quarter wave length Rochellesalt crystals having electrodes with electrical connections attachedthereto, the crystals being mounted by their ends upon a plurality ofthe faces and upon the thick portion of the means closing the upper endof said prism, a cup-shaped diaphragm of molded macerated Bakelite,having a thickness of one-quarter wave length in the material, securedby its edge to said plate and positioned to enclose with said plate allof said crystals and having its interior surfaces which are opposite thefree ends of said crystals only slightly spaced from them and acompressional wave conducting liquid filling the enclosed space.

3. An underwater sending or receiving device including means forradiating compressional waves, said means having six substantially planeradiating surfaces arranged one on each side of a hexagonal prism andhaving a Width of substantially 0.89 times the wave length of the wavesin the signaling medium.

4. An underwater sending or receiving device including means forradiating compressional waves, said means having six substantially planeradiating surfaces arranged one on each side of a hexagonal prism andhaving a width of substantially 0.89 times the wave length of the wavesin the signaling medium and a seventh radiating surface on top of saidprism having substantially the same width as said side surfaces.

5. An underwater electroacoustic device comprising a compressional wavetransmitting hollow casing, a hollow hexagonal prismatic mass memermeans mounting said member within said sing, a group of closelypositioned piezoelectric cystals having electrodes with electricalconnec- .tions attached thereto, the crystals being mounted upon each ofthe side faces of said member and projecting with their lengthssubstantially at right angles to the surfaces of the supporting prismfaces, the total width of each of said groups being not substantiallygreater than 0.89 times the wave length of compressional waves in thewater and a wave conducting liquid filling the casing.

6. An underwater electroacoustic device including a hexagonalprismaticmass member, a group of closely positioned piezoelectric crystals havingelectrodes with electrical connections attached thereto, the crystalsbeing mounted upon each of the side faces of said mass member andprojecting with their lengths substantially at'right angles to thesurfaces of the supporting prism faces, the 'total width of each of saidgroups being not substantially greater than 0.89 times the wave lengthof compressional waves in the water and means for conducting said wavesbetween the ends of the crystals and the water.

7. An underwaterv electroacoustic device comprising a compressional wavetransmitting hollow casing, a hollow hexagonal prismatic mass member,means mounting said member within said casing, a group of closelypositioned piezoelectric crystals having electrodes with electricalconnections attached thereto, the crystals being mounted upon each ofthe side faces of said member and projecting with their lengthssubstantial- 1y at right angles to the surfaces of the supporting prismfaces, each of said groups being substantially directed in a line normalto said surfaces and having dimensions with respect to the signalingwave length to form substantially a uniform radiation pattern in a planenormal to said surfaces.

8. An underwater electroacoustic device comprising a compressional wavetransmitting hollow easing having a comparatively rigid fiat base memberand a hollow diaphragm member, a hollow hexagonal prismatic mass member,means mounting said member at one end on said fiat base within saidcasing, a group of closely positioned piezoelectric crystals havingelectrodes with electrical connections attached thereto, the crystalsbeing mounted upon each of said faces of said member and projecting withtheir lengths substantially at right angles to the surfaces of thesupporting prism faces, each of said groups being substantially directedin a line normal to said surfaces and having dimensions proportionedwith respect to the signaling wave length to form substantially auniform radiation pattern in a plane normal to said surfaces.

l 9. An underwater electroacoustic device comprising a hollow casing, ahollow polygonal prismatic mass member having means closing the ends ofsaid mass member, means at one of said ends for mounting said memberwithin said casing, a plurality of electroacoustic transducer elementsmounted upon at least two of the faces of said mass member and acompressional wave-conducting liquid filling the casing.

10. An underwater electroacoustic device comprising a hollow casing. ahollow po ygonal 7 prismatic mass member having means closing the endsof said mass member, means at one of said ends for mounting said memberwithin said casing, a plurality of electroacoustic transducer elementsmounted upon at least two or the faces of said mass member, saidelements having a length projecting from the mass member of one-quarterwave length at the signaling frequency, the walls of the mass member inback of said electroacoustic elements having a thickness of oneeighth ofa wave length and said casing opposite .the ends of said electroacousticelements having a thickness of one-quarter wave length and beingcomposed of a. material having a specific acoustic impedance which isthe geometric mean between the impedance of said elements and theimpedance of the signaling medium and a compres- V sionalwave-conducting liquid filling the casing.

11. An underwater electroacoustic device comprising a' hollow casing, ahollow polygonal prismatic mass member having means closing the ends ofsaid mass member, means at one of said ends for mounting said memberwithin said casing, a plurality of piezoelectric crystals mounted uporiat least two of the faces of said mass member, said crystals having alength projecting from the mass member of one-quarter wave length at thesignaling frequency, the walls of the mass member in back of saidpiezoelectric crystals having a thickness of one-eighth of a wave lengthat the signaling frequency and said casing opposite the ends or saidpiezoelectric crystals having a thickness of one-quarter wave length andbeing composed of a material having a specific acoustic impedance whichis the geometric mean between the impedance of said crystals and theimpedance of the signaling medium and a compressional wave-conductingliquid filling the casing.

12. An underwater electroacoustic device comprising a, hollow casing, ahollow polygonal prismatic mass member having means closing the ends ofsaid mass member, means at one of said endsfor mounting said memberwithin said casing, .9, plurality of piezoelectric crystals havingelectrodes with electrical connections attached thereto mounted upon atleast two of the faces of said member and a compressionalwave-conducting liquid filling the casing.

BERTRAM M. HARRISON.

REFERENCES CITED UNITED STATES PATENT Name Date Nicolson Jan. 13, 1931Number

